Controlling spin-related material properties by electronic means is a key step towards future spintronic technologies. The spin Hall effect (SHE) has become increasingly important for generating, detecting and using spin currents, but its strength--quantified in terms of the SHE angle--is ultimately fixed by the magnitude of the spin-orbit coupling (SOC) present for any given material system. However, if the electrons generating the SHE can be controlled by populating different areas (valleys) of the electronic structure with different SOC characteristic the SHE angle can be tuned directly within a single sample. Here we report the manipulation of the SHE in bulk GaAs at room temperature by means of an electrical intervalley transition induced in the conduction band. The spin Hall angle was determined by measuring an electromotive force driven by photoexcited spin-polarized electrons drifting through GaAs Hall bars. By controlling electron populations in different (Γ and L) valleys, we manipulated the angle from 0.0005 to 0.02. This change by a factor of 40 is unprecedented in GaAs and the highest value achieved is comparable to that of the heavy metal Pt.

] Cavendish Laboratory University of Cambridge J J Thomson Avenue Cambridge CB3 0HE UK [2] Department of Physics University of Cyprus 1678 Nicosia Cyprus

] Cavendish Laboratory University of Cambridge J J Thomson Avenue Cambridge CB3 0HE UK [2] London Centre for Nanotechnology UCL 17 19 Gordon Street WC1H 0AH UK [3] PRESTO Japan Science and Technology Agency Kawaguchi 332 0012 Japan

] Institut fur Physik Johannes Gutenberg Universitat Mainz 55128 Mainz Germany [2] Institute of Physics ASCR v v i Cukrovarnická 10 162 53 Praha 6 Czech Republic

] Institute for Materials Research Tohoku University Sendai 980 8577 Japan [2] The Advanced Science Research Center Japan Atomic Energy Agency Tokai 319 1195 Japan [3] CREST Japan Science and Technology Agency Sanbancho Tokyo 102 0075 Japan

] Institute of Physics ASCR v v i Cukrovarnická 10 162 53 Praha 6 Czech Republic [2] School of Physics and Astronomy University of Nottingham Nottingham NG7 2RD UK

Cavendish Laboratory University of Cambridge J J Thomson Avenue Cambridge CB3 0HE UK

Institute of Physics ASCR v v i Na Slovance 2 182 21 Praha 8 Czech Republic

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